This invention relates generally to the waveguide art, and more specifically concerns a waveguide constructed from a laminate comprising several fibrous plies, wherein the fibers of the various plies are oriented in particular directions, selected so that the resulting waveguide is thermally compensated for a particular application.
In many waveguide applications, such as, for instance, in use with an antenna which comprises a plurality of radiating elements, a high degree of inter-element signal phase stability is required, i.e. the signals from the feeding waveguide present at all of the radiating elements must be in phase with each other. Many waveguides, while otherwise suitable for such applications, often cannot be used in a particular application because of such a phase instability characteristic.
The primary source of phase instability in waveguides is temperature sensitivity of the material comprising the waveguide. As the temperature of the environment changes, the length of the waveguide changes sufficiently relative to the length of the waveguide signal that the waveguide now accommodates additional cycles or a substantial portion of an additional cycle of the waveguide signal, which in turn results in a change of phase in the signal at the exit of the waveguide, and hence a change of phase in the signals applied to the radiating elements. Element to element phasing is thus seriously degraded.
Most waveguides have heretofore been constructed of metal, because of its good electrical properties. However, metals have a high coefficient of thermal expansion, and therefore are sensitive to changes in temperature, with resulting dimensional changes and phase instability for the waveguide.
Both active and passive compensation techniques have been used to increase the phase stability of such metal waveguides. In a representative active technique, circuitry is used to cause a time delay in the waveguide signal. The length of the delay is adjustable and can be changed to precisely compensate for the particular temperature change. However, such a system is not inherently corrective, i.e. it does not automatically change its correction as the temperature changes; it must instead be adjusted to each particular temperature. Such circuitry is also typically expensive to implement and requires installation.
Passive techniques generally involve the use of special materials which are not as temperature dependent as conventional metals. As an example, the metal Invar, which has a relatively low coefficient of thermal expansion, has been used. However, Invar is quite expensive, and also quite heavy, having a density of approximately that of steel. These characteristics make the widespread use of Invar in spacecraft waveguide applications impractical.
Another material which has been used for waveguides is graphite epoxy, which is a composite of graphite fibers and epoxy. The graphite has a negative coefficient of thermal expansion while the epoxy has a positive coefficient of thermal expansion. The use of a graphite epoxy composite has resulted in a decrease in temperature dependence of the waveguide by virtue of an improvement in the coefficient of thermal expansion by a factor of approximately two orders of magnitude. However, even such an improved performance resulting from the use of the graphite epoxy composite has proven to be insufficient for many applications in which an even higher degree of phase stability is required.
Accordingly, it is a general object of the present invention to provide a waveguide which overcomes one or more of the disadvantages of the prior art stated above.
It is a further object of the present invention to provide such a waveguide which is thermally compensated to the extent that it is relatively phase stable.
It is another object of the present invention to provide such a waveguide which is passively compensated for thermal expansion.
It is an additional object of the present invention to provide such a waveguide which is compatible with active circuitry designed for additional thermal compensation.
It is yet another object of the present invention to provide such a waveguide which is relatively lightweight and is competitive economically with other waveguide configurations.